I'm not sure what light you are refering to but I just had a very similar problem. Would not shift out of low gear-when this happened, the D4 light would be on. It didn't matter what gear you had the shifter in (if in reverse, D4 and reverse would be on, etc.) I found if I disconnected the battery, it would reset and would be fine for a few days. Anyway, it was the Automatic Transmission computer. Cheapest place I found one was from the salvage yard ($100). It is located under the passenger side floorboard. It did NOT show a trouble code. Hope this helps.